1. Field of the Invention
The present invention relates to an optical power splitter for splitting an input light beam into N equal output light beams, and more particularly, to a simple and economic integrated optical power splitter by which waveguide loss as can be reduced using a directional coupler, and equal outputs can be obtained, and a manufacturing method therefor.
2. Description of the Related Art
An optical power splitter splits an optical signal into a plurality of optical signals, and is used as a key component of an optical subscriber network. An optical power splitter may be manufactured by thermally joining two optical fibers together or grinding the side surfaces of the optical fibers and attaching the ground side surfaces to each other. This optical fiber splitter is a "1.times.2 optical power splitter" which splits a signal into two signals. The 1.times.2 optical power splitters are cascaded to make N outputs, thus forming a 1.times.N optical power splitter. Here, (2.sup.(logN/log 2)-1)1.times.2 optical power splitters are required. However, the optical power splitter using optical fibers is difficult to manufacture and has big differences in characteristics between manufactured devices, thus making it difficult to obtain N equal outputs. Also, since one device has a large volume, the structure of a 1.times.N optical power splitter requires a large integration area. Accordingly, much research has been conducted in an attempt to solve the above problem. It is known that the most practical technique to manufacture an optical power splitter is to use an integrated optics technique.
The integrated optics technique integrates several optical devices on a substrate based on an optical waveguide. If the integrated optics technique is used, optical devices are easily arranged, and many functional devices can be interconnected to each other in a narrow area. Therefore, the manufacturing costs can be reduced. FIG. 1A is a perspective view of a 1.times.2 optical power splitter manufactured by the integrated optics technique. An input channel optical waveguide is divided into two in a Y-shaped branch area. A slanting waveguide is used in the branching area to space between output optical waveguides, and the slanting waveguide is changed into a linear waveguide in a paralleling area. The angle .theta. (hereinafter called a branched angle) between slanting optical waveguides is usually very small, e.g., less than 1.degree.. The first reason why the branched angle is narrow is to reduce the scattering losses at the branching point. When a propagation path for optical waves is suddenly changed by the increase in the branched angle at the branching point, the optical waves cannot follow this path. As a result, larger scattering losses occur. The second reason is that in a 1.times.N optical power splitter constituted of cascaded 1.times.2 optical power splitters, if the branched angle is wide, the direction in which optical waves travel on the same phase surface is not consistent with the travel direction for the optical waveguide. In this case, an optical wave is biased in only one direction at the next branched point, so that equal division of 50:50 cannot be expected. FIG. 1B shows an example of a 1.times.8 optical power splitter formed by cascading 7 1.times.2 optical power splitters on a substrate. The 1.times.2 optical power splitters are connected to each other in a three-stage tree structure. On the first stage, input light is divided into two, on the next stage, the input light is divided into four outputs, and on the last stage, the input light is divided into eight outputs.
The length of a device is the most important feature in manufacturing the integrated 1.times.N optical power splitter. The length of the device is determined by the length of a branching region in each step of the 1.times.2 optical power splitter and the sum of the lengths of the paralleling regions. However, as described above, since the branched angle cannot be made small, the length of the device is determined by the length of the branching regions. In a 1.times.N optical power splitter consisting of slanting waveguides, the length of the device L.sub.N is expressed by the following Equation 1: ##EQU1##
wherein .theta. is a branched angle and an interval S is the distance between output waveguides.
It can be calculated from the Equation 1 that if the branched angle is 1.degree., then the length of a 1.times.32 optical power splitter having an S value of 250 .mu.m is 444 mm. However, it is actually difficult to manufacture such a long optical waveguide. Also, it is difficult to prepare a substrate for manufacturing this long waveguide, and even though the long waveguide is manufactured, propagation losses are large, so that this is not practical. Accordingly, in order to reduce the length of the device, an S-shaped curved optical waveguide has been tried instead of the slanting optical waveguide used in the branching area. However, as mentioned above, the propagation direction for optical waves on the same phase surface must be consistent with the propagation direction for optical waveguides to obtain an equal optical power division ratio. Thus, the radius of curvature of the curved optical waveguide cannot be short, thus resulting in a long curved optical waveguide. Accordingly, even when the optical power splitter uses an S-shaped curved optical waveguide, it is eventually difficult to shorten the length of the device.
Such a 1.times.N optical power splitter has the following problems.
First, the preparation of a substrate for manufacturing a device is difficult because of a long device, and large propagation losses occur.
Second, uniformity of the outputs is lowered. That is, manufactured individual devices are not uniform spatially due to a large ratio of width to length (device length/device width), thus lowering the uniformity of N outputs.
Third, it is costly to manufacture the device. That is, the manufacturing costs are increased by the inefficient use of a substrate. Since a typical substrate for manufacturing an optical waveguide device is circular, many portions of the substrate are wasted when a long device is manufactured. Also, when a division ratio of optical power is changed by a defect of a 1.times.2 optic power splitter at an upper stage occurring on account of the tree structure, this effect is continuously transmitted to 1.times.2 optical power splitters at the lower stage. Accordingly, even a small defect on an optical waveguide directly causes large errors. Thus, the yield is lowered. Furthermore, a very accurate V-shaped pattern at a Y-shaped optical waveguide branching point must be formed to equally split optical power, thus requiring a high processing cost to manufacture the accurate V-shaped pattern.